Conventional molecular beam epitaxy of monocrystalline group III-V semiconductor compounds is usually done with a condensed phase source for the group V element. (See A. Y. Cho and J. R. Arthur, Progress In Solid State Chemistry, edited by G. Somorjai and J. McCaldin, Pergamen Press, New York, 1975, Vol. 10, p. 157.) Usually this source is the solid element itself, although some work has been done in which the source of the group V elements is a solution of the group V elements in liquid group III elements.
A gaseous source for the formation of a group V molecular beam has been used in forming polycrystalline group III-V semiconductor layers. (See Morris, F. J. et al, "A New GaAs, GaP, and GaAs.sub.x P.sub.1-x Vacuum Deposition Technique Using Arsine and Phosphine Gas", Journal of Vacuum Science Technology, Vol. 11, No. 2, Mar./Apr. 1974.)
The solid elemental group V sources have several disadvantages. In the non-ideal effusion ovens used for MBE, solid group V sources are rapidly depleted and change surface area, and thus beam flux, with time. The nonconstant beam flux leads to difficulties in controlling the composition of mixed crystals. Solid phosphorus may cause beam intensity control problems because the solid red phosphorus source generally will consist of an ill-defined mixture of allotropic phases having different vapor pressures.
When the liquid solution of a group V element in a group III element is used as the group V source, the source is initially the group III-V compound in contact with the liquid as the result of preferential loss of group V element at the source temperature. At a given temperature, these sources will provide a constant flux of As.sub.2 or P.sub.2 as long as the solid group III-V compound remains. But, such sources are usually exhausted quickly because it is inconvenient and expensive to maintain a large volume of source group III-V compounds at the high temperature required. Furthermore, for such sources the group V element beam intensity cannot be varied independently of the group III element beam intensity.
Several studies have demonstrated that the quality of MBE material, at least for Al.sub.x Ga.sub.1-x As, improves with increasing substrate temperature during growth. [See for example, Casey, H. C., et al, IEEE Journal of Quantum Electronics, QE. 11, 467 (1975).] At higher substrate temperatures the efficiency with which the group V element is used decreases rapidly, as a result of its rapidly increasing partial pressure over the group III-V compound.
Accordingly, efforts have been directed at developing a method and apparatus for molecular beam deposition of group V elements, useful in forming monocrystalline and polycrystalline group III-V semiconductor layers, in which frequent replenishment of group V elements is not required and which produces an essentially constant beam flux.
Furthermore, there is an increasing interest in group III-V solid solutions including both As and P such as GaAs.sub.y P.sub.1-y or Ga.sub.x In.sub.1-x P.sub.y As.sub.1-y. Thus, a molecular beam source, which provides for precise control of the group V element beam intensities, is also needed.